The present invention relates to a distance measuring apparatus for use with cameras, video cameras, and the like for measuring the distance to an object.
One technique for the measurement of the distance to an object, the passive method, uses two light receiving sensors having the same optical system. These detect the contrast of an object in order to determine the distance in accordance with a shift in position caused by the parallax of the object as projected on each light receiving sensor. The passive method utilizes ambient light. Thus, it is possible to perform accurate distance measurement if the object is bright enough for the light receiving sensor to detect the object. However, the accuracy decreases when the object does not provide sufficient contrast or is dark.
Another technique for the measurement of the distance to an object, the active method, uses light emitted from a light projecting element towards the object, which is then reflected by the object and detected by means of light receiving elements. The position of the spectral center of the light reflected from the object is used to determine the distance by the triangulation method. With the active method, while distance measurement is possible when the object is dark, has no contrast, or has repeated patterns which reflect light, it is difficult to provide highly accurate range finding for an object located at a relatively large distance away due to a decrease in the quantity of light reflected from the object.
Prior art cameras have used either the passive or active methods for object distance measuring in cameras. As mentioned above, the passive and active distance measuring methods have advantages and disadvantages. Thus, a distance measuring apparatus which could implement both passive and active methods would be desirable in cameras so that highly accurate distance measuring could be obtained in response to various photo taking conditions.
With the active method, since reflected light, derived from a source of light, is detected for performing distance measurement, infrared light is used as the source of light. In this way, the light receiving elements can distinguish that light from ambient light. Thus, the active method allows for distinguishing the reflected light from ambient light or visible light. Consequently, the active method must necessarily use light receiving elements sensitive to infrared light. Thus, optical sensors formed in a line have normally been used as position sensing devices (PSDs).
With the passive method, since the contrast of an object is used for performing distance measurement, light receiving elements are used which are capable of outputting a light and dark contrast as a current value or voltage value, for example charge coupled devices (CCDs).
For this reason, it is necessary to have PSD and CCD light receiving elements, respectively, to facilitate the use of both active and passive methods. Moreover, it is also necessary to have additional optical systems associated with each method, which produces a more complex and larger constitution for the distance measuring apparatus. It is therefore difficult to provide a small and light-weight camera with such a distance measuring apparatus.
It is possible to envisage distance measuring based on both active and passive methods and which uses one of PSD or CCD sensors. However, the passive method will hardly function with PSDs since it is difficult to detect the contrast of the object while, due to the low sensitivity to infrared light of CCDs, it is difficult for CCDs to detect the reflected infrared light radiated from a source of light.
Thus, with the prior art, it has been found that in order to produce a distance measuring apparatus utilizing both methods and their consequent advantages, it has been necessary to constitute individual apparatus. For this reason, the constitution of the distance measuring apparatus has not been significantly simplified.
There is therefore a problem that the distance measuring apparatus can not be miniaturized, can not be reduced in weight, and can not provide any cost savings. Such apparatus can consequently not be applied to compact cameras. In this respect, since such cameras are typically equipped with automatic focusing (AF) devices, the mechanism of which occupies a large proportion of the volume and weight, incorporating a distance measuring apparatus with both methods into this type of camera is not beneficial in terms of improving miniaturization and weight reduction.
It is therefore an object of the present invention to provide a distance measuring apparatus using both the passive method and active method which is applicable to compact cameras.
According to the present invention there is provided a distance measuring apparatus which includes: at least three light-receiving sensors comprising CCD elements, an optical system for allowing the each light-receiving sensor to form the image of an object individually, a light-emitting element for radiating light to the object, a passive range finder comprising two light-receiving sensors as passive sensors among the light-receiving sensors, and the optical system, and an active range finder constituted by using the remaining one light-receiving sensor which is adopted as an active sensor, the optical system, and the light-emitting element.
In a particular embodiment, the present invention is constituted in a manner such that an infrared light cut-off filter is arranged on the light-receiving surfaces of the three light-receiving sensors and a visible light cut-off filter is arranged on the light-receiving surface of the active sensor, and the light-emitting element emits light of a range of wavelengths between the visible light region and infrared light region.
Conveniently, each passive and active sensor is preferably constituted as a line sensor having a number of CCD elements arranged in a line. In addition, the center of the line sensor in the longitudinal direction is preferably constituted as the active sensor, and both ends of the line sensor in the longitudinal direction are preferably constituted as the passive sensors.
Alternatively, the each sensor may be constituted by individually independent CCD elements for independent or synchronous drive control. Furthermore, the optical system may comprise passive lenses for forming the image of an object corresponding to the passive sensors, an active lens for forming the image of an object corresponding to the active sensor, and a condenser lens for condenser light emitted by the light-emitting element toward the object. Each lens may be preferably formed as one body.
The present invention provides that each sensor receives light of different wavelengths while the light emitting element is selected to emit light of an appropriate wavelength, whereby either one of the passive and active methods is available for distance measurement according to photo taking circumstances in spite of the sensors being constituted by CCD elements.
Thus, a need to have both PSD and CCD sensors is avoided so that the distance measuring apparatus can be simplified making it possible to realize miniaturization, a decrease in weight, and cost savings for distance measuring apparatus and cameras. Furthermore, use of the passive method is placed at a higher priority to provide distance measuring data under normal conditions, and, when reliable distance measuring data is not available by means of the passive method, the active method is performed to provide distance measuring data if the brightness of,the ambient light is less than a predetermined level. For this reason, reliable distance measuring data is available under various photo taking conditions.